It's a rule that the EU set for vehicles. EU-based manufacturers were the first to use r1234yf because of this rule, and American auto manufacturers followed suit to make it an industry standard, but there's no rule in the US other than the existing Clean Air Act, AFAIK.
The rule only affects newly manufactured vehicles as well, so there's no requirement to phase out R410A.
At this point most US vehicles use R134. Most R12 cars have been converted as once these things tend to leak, your ripe for a compressor rebuild, flush, oil swap, etc.
Why they set such hard GWP < 1 regulation? Isn't like < 50 enough? It's not emitted every time like CO2 emission of ICE but emitted at accident or poor deconstruction.
I don't know what was in their mind, but my guess is that it's due to automotive HVAC systems using flexible hoses rather than hard piping like stationary HVAC. The consequence of this is that those hoses aren't /truly/ impermeable, and it's commonplace for a vehicle HVAC system to need to be recharged after about 10 years of service life even with no "leaks". So, pretty much all gas going into the system will eventually end up in the atmosphere over time.
The rule only affects newly manufactured vehicles as well, so there's no requirement to phase out R410A.